1. Field of the Inventions
Embodiments of the present invention relate to thin film gallium (Ga) electroplating methods and chemistries employing electrolytes or solutions comprising mixtures of water and certain classes of organic liquids. Such films have application in the field of electronic devices such as solar cells.
2. Description of the Related Art
Thin film solar cells have attracted much attention lately because of their potential low cost. Thin film solar cells may employ, as their light absorbing layer or absorber, polycrystalline silicon, amorphous silicon, cadmium telluride (CdTe), copper indium gallium selenide (sulfide) (CIGS(S)), etc. The processing methods used for the preparation of thin film solar cell absorber layers can generally be classified as dry and wet processes. The dry processes include physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques, which are usually well developed, however, expensive. Wet processes include ink spraying or printing, chemical bath deposition (CBD) and electrochemical deposition (ED), also called electrodeposition or electroplating. Among these methods, CBD is popular for the preparation of some n-type semiconductor films like CdS, ZnSe, In—Se, etc. In ink deposition processes, inks comprising nano-particles dispersed in a solvent are deposited on a substrate. When the solvent evaporates away, it leaves behind a precursor layer comprising the nano-particles. The precursor layer is then sintered at high temperatures to form the absorber.
Electrochemical deposition techniques can provide thin precursor films which may then be converted into solar cell absorbers. One recent application of electroplated copper (Cu), indium (In) and gallium (Ga) films is in the formation of Cu(In,Ga)(Se,S)2 or CIGS(S) type layers, which are the most advanced compound absorbers for polycrystalline thin film solar cells. It should be noted that the notation (In, Ga) means all compositions from 100% In and 0% Ga to 0% In and 100% Ga. Similarly, (Se,S) means all compositions from 100% Se and 0% S to 0% Se and 100% S. Applying electrodeposition to the formation of a CIGS(S) type absorber layer may involve a two-stage or two-step processing approach comprising a precursor deposition step and a reaction step. A thin In layer, for example, may be electroplated on a Cu layer. A thin Ga film may then be formed on the In layer to form a Cu/In/Ga stack precursor. The Cu/In/Ga precursor stack thus obtained may then be reacted with selenium (Se) to form a CIGS absorber. Further reaction with sulfur (S) would form a CIGS(S) layer. The CIGS(S) absorber may be used in the fabrication of thin film solar cells with a structure of “contact/CIGS(S)/buffer layer/TCO”, where the contact is a metallic layer such as a molybdenum (Mo) layer, the buffer layer is a thin transparent film such as a cadmium sulfide (CdS) film and transparent conductive oxide (TCO) is a transparent conductive layer such as a zinc oxide (ZnO) and/or an indium tin oxide (ITO) layer.
In a thin film solar cell employing a Group IBIIIAVIA compound absorber such as CIGS(S), the cell efficiency is a strong function of the molar ratio of IB/IIIA. If there are more than one Group IIIA materials in the composition, the relative amounts or molar ratios of these IIIA elements also affect the properties. For a Cu(In,Ga)(S,Se)2 or CIGS(S) absorber layer, for example, the efficiency of the device is a function of the molar ratio of Cu/(In+Ga), where Cu is the Group IB element and Ga and In are the Group IIIA elements. Furthermore, some of the important parameters of the cell, such as its open circuit voltage, short circuit current and fill factor vary with the molar ratio of the IIIA elements, i.e. the Ga/(Ga+In) molar ratio. In general, for good device performance Cu/(In+Ga) molar ratio is kept at or below 1.0. For ratios close to or higher than 1.0, a low resistance copper selenide phase may form, which may introduce electrical shorts within the solar cells. As the Ga/(Ga+In) molar ratio increases, on the other hand, the optical bandgap of the absorber layer increases and therefore the open circuit voltage of the solar cell increases while the short circuit current typically may decrease. It is important for a thin film deposition process to have the capability of controlling both the molar ratio of IB/IIIA, and the molar ratios of the Group IIIA components in the composition. Therefore, if electrodeposition is used to introduce the Ga into the film composition, it is essential that the electroplated Ga films have smooth morphology and be free of defects such as pinholes. It should be noted that the typical thickness of Ga layers to be electroplated for CIGS(S) absorber formation is in the range of 50-300 nm and many prior art electroplated Ga layers display a peak-to-valley surface roughness in the range of 50-500 nm, which means that these films are very thick in some areas and very thin in others.
In an application of electroplated Ga layers to solar cell fabrication, the Ga layer may be electroplated to form precursor stacks with structures such as Cu/In/Ga, Cu/Ga/In, etc. These stacks may then be reacted at high temperature (typically in the range of 400-600° C.) with a Group VIA material such as Se and S to form a CIGS(S) absorber layer. The absorber layer may then be further processed to construct a solar cell. US Patent Application with publication No. 20070272558, entitled “Efficient Gallium Thin Film Electroplating Methods and Chemistries” filed by the applicants of this application and incorporated herein by reference, discloses new methods and chemistries to deposit Ga films with high plating efficiency. Other work on electrodeposition of Ga includes the publication by S. Sundararajan and T. Bhat (J. Less Common Metals, vol. 11, p. 360, 1966) who utilized electrolytes with a pH value varying between 0 and 5. Other researchers investigated Ga deposition out of high pH solutions comprising water and/or glycerol. Bockris and Enyo, for example, used an alkaline electrolyte containing Ga-chloride and NaOH (J. Electrochemical Society, vol. 109, p. 48, 1962), whereas, P. Andreoli et al. (Journal of Electroanalytical Chemistry, vol. 385, page. 265, 1995) studied an electrolyte comprising KOH and Ga-chloride. While some of these previous works used very corrosive solutions, i.e., pH-15, most of them were carried out under low plating efficiencies in low pH electrolytes, the plating efficiencies being typically 20% or lower. Glycerol, due to its high boiling temperature has also been used in high temperature (>100° C.) preparation of electrodeposition chemistries to plate molten globules of Ga—In alloys (see e.g. U.S. Pat. No. 2,931,758). Although, glycerol-based plating solutions may be adequate to obtain Ga deposits in the form of thick molten globules such deposits cannot be used in the formation solar cell absorbers such as thin film CIGS(S) compounds. From the foregoing, there is a need to develop Ga electrolytes and electrodeposition methods to generate smooth, uniform and defect-free Ga thin films with high plating efficiencies on surfaces of varying chemical composition. This way Ga layers may be electroplated onto different cathode surfaces for electronics applications, specifically for the fabrication of high quality CIGS(S) type thin film solar cell absorbers.